Gelled aqueous treating fluids are often used to treat subterranean zones penetrated by well bores. For example, gelled aqueous treating fluids are commonly used in hydraulic fracturing operations carried out in subterranean zones to increase the flow of oil and natural gas therefrom.
In hydraulic fracturing operations, a viscous fracturing fluid such as a gelled aqueous fluid is injected into a subterranean zone penetrated by a well bore at a rate and pressure sufficient to create one or more fractures in the zone. Continued pumping of the viscous fracturing fluid extends the fractures and proppant particles such as sand or other particulate material are deposited in the created fractures. The proppant particles function to hold the fractures open after reduction of the pressure applied to the fractures, thus providing conductive channels through which produced fluids can readily flow to the well bore.
To adequately form and propagate fractures in subterranean formations, the fracturing fluid must have sufficient viscosity to create and extend the fractures and to retain proppant particles in suspension. In order to provide viscosity to aqueous fracturing fluids, hydratable gelling agents such as polysaccharide polymers are added to the fluids. The viscosity can be increased further by adding crosslinking agents to the fluids. Examples of conventional crosslinking agents that can be used include borate ions or polyvalent metals such as titanium or zirconium that form chemical bonds between the viscosifying gelling agent molecules and increase the viscosity of the fracturing fluid.
Seawater, brines and hard water contain significant quantities of divalent cations such as Ca+2 and Mg+2. The use of these water sources for crosslinked gelled treating and fracturing fluids is complicated due to the precipitation of metal hydroxides at the pH range the fluids are most effective. In addition, the effectiveness of borate cross-linked systems for increasing viscosity is reduced when seawater, brines or hard water is used. This is due to the divalent metal ions inhibiting the borate cross-linked gelling agent.
To overcome these problems and allow the use of seawater, brines and hard water, conventional metal ion chelating agents have been utilized. The chelating agents include inorganic polyphosphates, amino polycarboxylic acids, salts of polycarboxylic acids, polyacrylates, and polyphosphonated chelating salts of agents such as diethylenetriaminepenta-(methylene phosphonic acid), salts of nitrolotrimethylenephosphonic acid, salts of ethylenediamine hydroxydiphosphonic acid and salts of ethylenediamine tetramethylene phosphonic acid. These chelating agents have been used to aid borate crosslinking by sequestering the metal ions interfering with the borate crosslinking.
While the chelating agents are generally effective, they are not environmentally benign and environmental regulations often restrict their use.
Thus, there are needs for improved methods of treating subterranean zones and gelled aqueous treating fluids containing environmentally benign sequestering agents for sequestering divalent metal ions in seawater, brine or hard water.